The present invention is directed to voltage regulating transformers, and more particularly to a controller which enables a load tap changing transformer to be controlled by means of a standard voltage regulator control device.
Transformers for providing a controlled output voltage can generally be classified as one of two types. One type, simply known as a voltage regulating transformer, is used in applications having low voltage and power requirements. The other type of transformer, a load tap changing (LTC) transformer, is generally a much larger device and is used in power transmission. These different types of transformers have different modes of operation and different control requirements. A voltage regulating transformer has small components and utilizes a small motor to adjust the output voltage. As a result, the controller for a voltage regulating transformer can be relatively simple in structure and operation. One example of a controller for a voltage regulating transformer is disclosed in U.S. Pat. No. 4,419,619.
In contrast, an LTC transformer is a much more complex and difficult device to control. For example, it is required to regulate a three-phase power supply. In addition, the input parameters that must be considered to provide proper control are different from those of a voltage regulating transformer controller. As a result, the controller for an LTC transformer is much more complicated and expensive than that for a voltage regulating transformer.
Controllers for voltage regulating transformers are in widespread use. As shown by the aforementioned U.S. patent, advances in controllers for voltage regulating transformers have kept pace with technology, and today's controllers are microprocessor-based devices which can be readily programmed to provide desired functions.
It would be desirable to utilize such a controller, which is designed for a voltage regulating transformer, to control the operation of an LTC transformer. However, because of the different types of control signals and input sensing that are required for an LTC transformer, it is not possible to use a standard voltage regulator control for the operation of an LTC transformer. For example, in many power transmission applications, it is desirable to connect multiple transformers, which receive power from different respective sources, in parallel to a single transmission line. With this type of arrangement, it is necessary to ensure that all transformers are operating at the same output voltage. Otherwise, if one transformer has a lower output voltage than the others, it will function as a current sink rather than contribute its proportionate share of power to the load. Accordingly, the control of an LTC transformer involves a procedure identified as current balance paralleling, in which the circulating current (reactive current) in the transformer is measured to be sure that a proper balance is maintained among all of the transformers connected to the line. A standard voltage regulator controller does not measure circulating current, and therefore is not able to perform this required procedure.
Another difference between an LTC transformer and a voltage regulating transformer is the fact that, due to the higher currents it conducts, the LTC transformer employs a vacuum interrupter to make and break the load current during a tap change. The controller for an LTC transformer must be able to detect a faulty vacuum switch, and reverse a tap change operation if such a fault is detected. A standard voltage regulator controller does not include these capabilities.
In the past, rather complex mechanisms which included external relays and timers were used to carry out the necessary control functions required by the various modes of operation of an LTC transformer. It is desirable, however, to provide a device which allows controllers for voltage regulating transformers to be employed with LTC transformers, and thereby simplify the control of LTC transformers while eliminating the need for complex external control mechanisms.